A dry-type solar cell using a semiconductor, such as silicon, is on the stage of being practically used. A semiconductor solar cell has high generation efficiency, but is expensive due to the use of a highly purified material.
As a relatively inexpensive solar cell, there is a wet-type solar cell using titanium dioxide (TiO2) and an electrolyte.
A titanium dioxide solar cell arrangement is described with reference to FIG. 1.
FIG. 1(a) shows a titanium dioxide solar cell having a basic arrangement, and FIG. 1(b) shows an improved titanium dioxide solar cell called a dye sensitized type. In the titanium dioxide solar cell having the basic arrangement shown in FIG. 1(a), numeral 1 represents a glass substrate forming on one surface thereof a transparent conductive layer 2 made of FTO or the like, and which serves as a photoelectrode. Numeral 3 represents a porous titanium dioxide sintered material. Numeral 4 represents an electrolytic solution, and an iodine electrolyte having iodine dissolved in an aqueous potassium iodide solution is generally used. Numeral 5 represents a platinum counter electrode which is formed on a glass substrate 7 having formed thereon a conductive layer 6 made of FTO or the like. Numeral 8 represents a sealing material, and numeral 9 represents an external load, such as a resistor.
The incident light which has passed through the transparent conductive layer 2 on the glass substrate 1 is absorbed by the porous titanium dioxide sintered material 3. The porous titanium dioxide sintered material 3 which has absorbed the light is electronically changed from the ground state to an excited state, and the excited electrons are caused to go out of the transparent conductive layer 2 due to diffusion, and introduced through the load 9 from the transparent conductive layer 6 to the platinum counter electrode 5.
However, the light which the titanium dioxide can utilize in electric generation is only ultraviolet light having the wavelength of 380 nm or less, and the ultraviolet light in this range of wavelength is only 4% of the sunlight, and thus the sunlight utilization efficiency is 4% at most, practically 1%, and therefore, this solar cell using titanium dioxide exhibits extremely poor utilization efficiency of the sunlight.
For removing the drawback of titanium dioxide that the usable wavelength range of light is narrow, there has been known a dye sensitized solar cell (DSSC) which has sintered porous titanium dioxide having a ruthenium complex dye adsorbed thereon, and which thereby can use light in the visible light region that is longer in wavelength than the ultraviolet light.
The basic arrangement of the dye sensitized solar cell is described with reference to FIG. 1(b).
In this figure, numeral 1 represents a glass substrate forming on one surface thereof a transparent conductive layer 2 made of FTO or the like. Numeral 3 represents a porous titanium dioxide sintered material having a ruthenium complex dye adsorbed on the porous surface thereof. Numeral 4 represents an electrolytic solution, and an iodine electrolyte having iodine dissolved in an aqueous potassium iodide solution is generally used. Numeral 5 represents a platinum counter electrode which is formed on a glass substrate 7 having formed thereon a conductive layer 6 made of FTO or the like. Numeral 8 represents a sealing material, and numeral 9 represents an external load, such as a resistor.
The incident light which has passed through the FTO transparent conductive layer 2 on the glass substrate 1 is absorbed by the ruthenium complex dye adsorbed on the porous surface of the porous titanium dioxide sintered material 3. The ruthenium complex dye which has absorbed the light is electronically changed from the ground state to an excited state, and electrons in the excited state in the ruthenium complex dye are injected into the porous titanium dioxide sintered material 3, so that the ruthenium complex dye changes to an oxidation state. In this instance, for effectively injecting the excited electrons in the ruthenium complex dye into the porous titanium dioxide sintered material 3, the excitation energy level of the ruthenium complex dye must be lower than the conduction band energy level of the porous titanium dioxide sintered material 3 which is a semiconductor. The electrons injected into the porous titanium dioxide sintered material 3 are caused to go out of the transparent conductive layer 2 due to diffusion, and introduced through the load 9 to the platinum counter electrode 5. On the other hand, the oxidized ruthenium complex dye receives electrons from iodine contained in the iodine electrolyte 4 and is changed back to the ruthenium complex dye in the ground state.
A dye sensitized solar cell having the above-mentioned arrangement theoretically has sunlight utilization efficiency of 30%, but, practically 10% at most.
Titanium dioxide has a photocatalytic function, and, as a material similarly having the photocatalytic function, fused quartz treated with halogen acid is described in JP-A-2004-290748 and JP-A-2004-290747.
As a material similarly having a photocatalytic action, synthetic quartz treated with hydrofluoric acid is described in International Patent Application Publication No. WO2005/089941.
The synthetic quartz photocatalyst functions as a photocatalyst in a wavelength range of 200 to 800 nm which is further wider than the range for the photocatalyst using fused quartz as a raw material described in JP-A-2004-290748 and JP-A-2004-290747.
The present inventors have found that synthetic quartz or fused quartz, which is silicon dioxide, has a photovoltaic ability, and have proposed the silicon dioxide solar cell described in International Patent Application Publication No. WO2011/049156.
The solar cell described in International Patent Application Publication No. WO2011/049156 is described with reference to FIG. 2.
In FIG. 2, numerals 11 and 17 represent 30 mm×30 mm glass substrates having a transparent conductive layer FTO (fluorine-doped tin oxide) layer 12 and an FTO layer 16, respectively, formed thereon, and the solar cell has a size of 20 mm×20 mm.
An n-type semiconductor layer 13 of zinc oxide (ZnO), titanium dioxide (TiO2), or the like is formed on the FTO layer on the light incident side, and a platinum layer 15 is formed on the FTO layer 16 positioned opposite to the FTO layer 12 on the light incident side.
A solar cell material 20 having the thickness of 0.15 to 0.20 mm and having a mixture of glass containing SiO2 and an electrolyte is sealed between the n-type semiconductor layer 25 and the platinum layer 26.
With respect to the solar cell material 27, there is used one which is obtained by immersing glass particles containing SiO2 or the like in a 5% aqueous solution of hydrofluoric acid for 5 minutes, washing the particles with water, drying them, and pulverizing the resultant particles so that the particle diameter becomes 0.2 mm or less.
The electrolyte is obtained by adding 0.1 mol of LiI, 0.05 mol of I2, 0.5 mol of 4-tert-butylpyridine, and 0.5 mol of tetrabutylammonium iodide to 0.5 mol acetonitrile solvent.
While the details of the mechanism of silicon dioxide photocell are unclear, there is a phenomenon that when silicon dioxide is irradiated with the sunlight having the wavelength of 200 to 800 nm, the light is absorbed and electrons flow from the electrode on the silicon dioxide side toward the counter electrode through a load, in other words, a current flows from the counter electrode toward the electrode on the silicon dioxide side.
As a solar cell material, synthetic quartz is the most useful, but fused quartz glass, soda-lime glass, non-alkali glass, or borosilicate glass can also be used in electric generation.
The short-circuit current and open circuit voltage obtained when irradiated with a light from a fluorescent lamp at 15,000 to 19,000 lux are as follows:
short-circuit currentopen circuit voltageSynthetic quartz:0.5 μA35 mVFused quartz glass:0.5 μA30 mVSoda-lime glass:0.3 μA15 mVNon-alkali glass:0.4 μA30 mVBorosilicate glass:0.3 μA14 mV
Further, even with respect to the silicon dioxide composition which is not treated with hydrofluoric acid, the following short-circuit current and open circuit voltage have been obtained.
short-circuit currentopen circuit voltageSynthetic quartz:0.1 μA3 mVFused quartz glass:0.2 μA3 mVSoda-lime glass:0.1 μA5 mVNon-alkali glass:0.1 μA5 mVBorosilicate glass:0.2 μA12 mV 